1. Technical Field
Several aspects of the invention relate to a data transfer circuit that transfers data stored in an external memory. Further, several aspects of the invention relate to a semiconductor integrated circuit equipped with such a data transfer circuit.
2. Related Art
Apparatuses that use a display device such as an LCD display device (for example, cellular phone devices, personal computers and the like) display blinking icons, animated icons and the like, for indicating that, for example, a battery has a few remaining capacity, an external device such as a printer, a hard disk device or the like is being accessed, etc.
In general, image data for displaying an image is stored in a memory (frame memory). Addresses of a frame memory have one-to-one correspondence to positions (pixels) on a display screen of a display body (for example, an LCD panel), and a display controller (for example, an LCD controller) transfers the image data stored in the frame memory to the display body, whereby the image is displayed on the display screen of the display body. Generally, the display body does not have a function to store image data. Therefore, the display controller repeatedly transfer image data stored in the frame memory to the display body periodically (cyclically) according to the frame rate.
For example, the blinking display of an icon can be realized through rewriting image data in a frame memory at a longer cycle than the frame rate. For example, a blinking display of a letter (character) such as “◯” can be realized as follows. First image data that has a character “◯” disposed therein and second image data that does not have the character “◯” are prepared in a memory region (work area) other than the frame memory, and the first image data and the second image data are alternately transferred to the frame memory according to a desired blinking display time interval. Also, an animated display can be realized as follows. Many more image data sets are prepared in a work area, and some or selected ones of the image data sets are transferred to the frame memory according to a desired time interval.
In prior art, in the above-described operation of transferring image data to the frame memory, a timer circuit measures an interval (timing) to change the display, the timer circuit informs the CPU of the timing to change the display with an interrupt signal or the like, and the CPU rewrites the image data in the frame memory. However, according to the prior art technology, the CPU has to perform an interrupt processing (a processing to rewrite the image data in the frame memory) while temporarily stopping a processing being conducted at the time of generation of the interrupt.
Japanese Laid-open Patent Application JP-A-11-296472 describes a display control circuit having a memory device that stores data for cyclical display and a transfer device that performs DMA transfer of the data to a predetermined display device, wherein the transfer device repeats DMA transfers in synchronism with a periodic signal corresponding to a predetermined cycle. By this display control circuit, image data for blinking display or the like can be DMA-transferred, such that it becomes unnecessary for the CPU to perform an interrupt processing each time a blinking display or the like takes place.
However, this display control circuit can perform only predetermined (designed) DMA transfers. Therefore, for example, when the number of icons to be blinked wants to be increased, there is a problem in that the hardware needs to be reconfigured, and thus it is not easy to expand the display function.